1. Field of the Invention
This invention relates generally to an optical system for correcting wavefront phase aberrations of a light beam and, more particularly, to an optical system for correcting wavefront phase aberrations of a light beam that employs a process of interfering a reference beam with the main beam to identify destructively interfering locations in the wavefront of the main beam that are then removed from the beam using a light valve.
2. Discussion of the Related Art
Certain types of optical transmission systems, such as optical communications systems, imaging systems, etc., transmit a coherent light beam carrying information through a medium, such as air. Because the light beam is coherent, the phase of the beam is substantially constant across the beam wavefront when it is generated. However, the medium typically corrupts the beam by introducing distortions to the beam that causes portions of the wavefront to have a different phase than other portions of the wavefront at any given instant in time. If this wavefront phase aberration was not corrected at the receiver, the light beam could not be effectively focused onto receiving optics, such as a fiber optic cable, and thus a significant intensity of the beam would be lost. Therefore, it is known in the art to correct wavefront aberrations at the receiver of an optical system of this type.
Different systems are known in the art to correct wavefront aberrations in an optical system of the type being described herein. Typically, these types of systems employ a wavefront sensor, such as a Schack-Hartman sensor, that measures the tilt of individual portions of the beam wavefront. A reconstructor is used to generate a surface representative of the phase relationship of the beam wavefront, and a deformable mirror is used to generate a compliment of the surface generated by the reconstructor to provide a corrected beam. The deformable mirror typically includes a plurality of actuators positioned behind the mirror that act to deform the mirror at the desired locations to provide the compliment of the sensed wavefront to correct the phase. This process is performed many times a second depending on the particular application.
In one known system, a wavefront sensor measures the tilt of wavelets within the main optical beam at a plurality of locations across the beam wavefront to determine the tilt or slope of the beam wavefront. The article Fried, David L., xe2x80x9cLeast-square fitting a wave-front distortion estimate to an array of phase-difference measurements,xe2x80x9d J. Opt. Soc. Am., Vol. 67, No. 3, March, 1977, pgs. 370-375, discusses one of the first known techniques for providing tilt measurements of a beam wavefront. In one design, the light is directed through an array of lenslets that focus individual portions of the wavefront in the direction determined by their slope. Algorithms are then employed in a wavefront reconstructor that reconstruct the phase of the wavefront to correct the phase by minimizing the least squares error between the observed phase gradient (tilt) and its computed value.
Another other known technique for providing beam wavefront sensing includes determining intensity measurements of the wavefront using curvature sensors. The article Roddier, Francois, xe2x80x9cCurvature Sensing and Compensation: A New Concept in Adaptive Optics,xe2x80x9d Applied Optics, Vol. 27, No. 7, Apr. 1, 1988, pgs. 1223-1225 provides one of the original discussions on determining curvature measurements of a beam wavefront. To determine the curvature intensity measurements, algorithms are employed to derive the curvature (Laplacian) of the phase of the wavefront directly by differences in ratios of the measured intensities of the wavefront. The phase is then recovered by solving Laplace""s equation with the right hand side equal to the measured curvature.
The various optical systems used to reconstruct a beam wavefront to reduce or eliminate aberrations suffer from a number of drawbacks. For example, the wavefront sensor, beam reconstructor and deformable mirror used in these systems are typically very expensive devices that add significant cost to the overall optical system. Further, because the various actuator devices used to adjust the deformable mirror are limited in size, systems having larger optics typically cannot be accommodated by these wavefront reconstruction systems because of resolution limitations.
What is needed is a beam wavefront reconstruction system for correcting the phase of a beam wavefront that is less expensive than the known systems in the art, and can provide greater resolution. It is therefore an object of the present invention to provide such an optical system.
In accordance with the teachings of the present invention, an optical system for correcting wavefront aberrations of an optical beam is disclosed. In one embodiment, the beam is received by an entrance pupil, where a portion of the beam at the entrance pupil is coupled off of the main beam to be used as a reference beam. The reference beam is taken from a small enough portion of the main beam so that the temporal coherence of the reference beam wavefront is substantially in phase. The reference beam is amplified by a coherent optical amplifier, expanded and collimated. The main beam and the collimated reference beam are applied to a beam splitter that splits the main beam and the collimated reference beam into two separate beam paths, where the main beam and the reference beam traveling along a common path are coupled together. One optical path from the beam splitter is received by a detector array and the other optical path from the beam splitter is sent through a light valve. Those portions of the main beam and the reference beam that are more than 90 degrees out of phase with each other destructively interfere at the detector array. The outputs of the detectors in the array are measured by threshold electronics. The threshold electronics cause the light valve to attenuate the portions of the beam on the other path that represent the destructively interfering portions of the main beam and the reference beam, so those portions do not pass through the light valve. Therefore, only those portions of the main beam that are within 90xc2x0 in phase with the reference beam continue on through the receiver optics.
In an alternate embodiment, the output beam is used as the reference beam in a closed loop manner. Further, in yet another embodiment of the present invention, the coherent optical amplifier is eliminated, and the beam splitter is an unbalanced beam splitter so that the magnitude of the main beam and the reference beam at the detector array is substantially equal.
Additional objects, advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.